Drones that collide with planes cause more damage than birds of the same size because of their solid parts such as motors and batteries.

That’s according to a study released by the Federal Aviation Administration on Tuesday.

The report’s researchers say aircraft-manufacturing standards designed for bird strikes aren’t appropriate for ensuring planes can withstand collisions with drones.

Reports of close calls between drones and airliners have surged. Canadian officials say a drone hit a small charter plane last month over Quebec City, the first such incident in Canada. The plane landed safely.

A team of researchers from four universities used computers to simulate collisions between drones weighing 2.7 to 8 pounds and common airliners and business jets. In some cases, drones would have penetrated the plane’s skin.